Various types of machines utilize a movable boom or arm having disposed on the end thereof a workhead or other function-performing device. An example of such a machine is a material handler which includes a boom having a workhead in the form of a set of pallet forks. The length of the boom and the angle of the boom with respect to a reference plane, such as a horizontal plane, are independently adjustable by means of actuators, such as rams or cylinders, in response to operator input to a control system. Such a control system is capable of polar coordinate operation or control, since the independent control over each actuator renders the end of the boom easily positionable in terms of a polar coordinate system defined by the parameters of boom length and boom angle.
One consequence of this polar coordinate control is that it is very difficult to obtain straight line movement of the end of the boom along an axis other than a line coincident with the longitudinal axis of the boom. This is due to the fact that the boom length and boom angle must be continuously varied in a synchronized fashion if such straight line movement is desired. It is extremely difficult, if not impossible, for an operator to obtain the necessary control over each of the actuators in order to effect such straight line movement, for example, when it is desired to move the above-described pallet forks horizontally to insert the forks under a pallet.
In order to overcome the above problem, manufacturers of machines which utilize such booms or arms have resorted to mounting the entire machine on rails between the machine body and the tracks or wheels thereof so that straight line movement of the end of the boom can be accomplished by moving the entire machine. This, obviously, is expensive and only affords straight line motion in a horizontal plane, assuming the machine is level. This is, therefore, a poor solution to the problem.
One type of control for moving the end of a robot arm in approximately a straight line is disclosed in Devol U.S. Pat. No. 3,543,910. This control system includes a control program which operates actuators for varying the arm angle and extension. The control program operates one of the actuators to cause an incremental displacement of the angle of the arm with respect to the horizontal. The system includes mechnical feedback apparatus for sensing this incremental displacement of the arm angle. The control program is responsive to this sensed incremental change in angle and controls the remaining actuator to change the arm length so that the end of the arm is moved in a path which is a stepwise approximation of a straight line.
Devol does not disclose the ability to use manual input to accurately control the travel of the end of the arm. This capability to respond to an input other than a preprogrammed input is essential in most applications. Moreover, the Devol control exerts only intermittent control over the actuators which in turn control the displacement of the end of the arm, and hence this control cannot produce true straight line movement.